CMake: Remove LLVM_DYLIB_SYMBOL_VERSIONING

Summary:
This option is no longer needed since r300496 added symbol
versioning by default

Reviewers: sylvestre.ledru, beanz, mgorny

Reviewed By: mgorny

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D49835

llvm-svn: 338751